Type what a sealed box cost you, how many packs it holds and how many cards are in each pack. You get the cost of each pack and each card. Add what your pulls are worth to see them against the box.
What you paid, with tax and shipping.
Your own total for the cards you opened.
When you open a box, its price becomes the cost of what came out. If you later sell one of those cards, the cost you count against the sale is its share of the box, not zero. Cost per card is the even split; it is the simplest fair starting point.
An even split gives a common card and the chase card the same cost. Many collectors who sell split the box by what each card is worth instead, so the hit you sell carries most of the cost and the bulk carries little. Either way, the costs of everything you kept add back up to what you paid for the box.
What you paid, the date and where you bought it are the record behind every number here. See taxes for card sellers for why that record matters when you sell.